1. Field of the Invention
The invention relates to a video system conversion method and circuit. More particularly, it relates to a video system conversion method and circuit for converting a signal of one video system to a signal of another video system. Still more particularly, the present invention relates to a video system conversion method and circuit for converting a signal produced in an RGB video system, which is used in personal computers, to an NTSC signal, which is used for television.
2. Description of the Prior Art
Recent video display technology has made it possible to display the video data of a personal computer on a screen of a television receiver. The scanning system (Variable Graphics Array method, referred to as "VGA" below) used for a personal computer is different in a manner discussed below from television systems (there are two television systems, the NTSC system and the PAL system: the NTSC system is used for the explanation below). In order to display the video data according to two different systems, the scanning system used in a personal computer and the system used in a television, a conversion of the signals is necessary. The difference between the scanning system for a personal computer display and that for a television system is briefly explained below. In the scanning system used for a personal computer, the video data are sequentially scanned and displayed from the top line to the bottom line, from the left to the right, in respective lines on the display. On the other hand, in the scanning system used for a television, the video data are scanned by skipping every other line, which is referred to as interlaced scanning. In this interlaced scanning system, odd-numbered first field scanning lines (the first line, third line, fifth line, . . . , the five-hundred-and-twenty-fifth line) of the 525 lines are firstly displayed on the screen, then, even-numbered second field scanning lines (the second line, fourth line, sixth line, . . . , five-hundred-twenty-fourth line) are secondly displayed on the screen.
FIG. 24 is a conceptual drawing showing the conversion of video data from the VGA system to the NTSC system. In FIG. 24, the left column shows scanning in the VGA system, and the right column shows scanning in the NTSC system. In order to convert the video data of the VGA system to that of the NTSC system, only the odd-numbered scanning lines in the first VGA screen are used as the scanning lines of the first field in the NTSC system, and the even-numbered scanning lines in the second VGA screen are used as the scanning lines of the second field in the NTSC system. In other words, two screens of the VGA system are used to construct one screen in the NTSC system.
FIG. 23 shows a conventional video system conversion circuit disclosed in Laid-open Japanese patent application No. 8-242427. In FIG. 23, input RGB signals of the VGA system are converted into an intensity signal Y, and chrominance signals (B-Y) and (R-Y) in the NTSC system. The chrominance signals can be represented in three systems: a system which uses an I signal and a Q signal, a system which uses a U signal and a V signal, and finally the system which uses an intensity signal Y, and chrominance signals (B-Y) and (R-Y) which is mentioned above. These three systems are not substantially different from each other, and the only difference is in the calculation factors used for calculating the chrominance signals from RGB signals. The present invention may be used for any of these three systems of representation of the chrominance signals. Although the invention is explained below using the intensity signal Y and the chrominance signals (B-Y) and (R-Y) consistently, the present invention may be used for the other two systems in exactly the same manner. The circuit of FIG. 23 includes input terminals 101, 102, and 103 for receiving RGB signals of the VGA system, and these input terminals receive an R signal (red signal), a G signal (green signal), and a B signal (blue signal), respectively. RGB signals comprise one pixel with three signals. Since the respective R signal, G signal, and B signal contain intensity components and chrominance components, the data include more than the signals of the NTSC system. An analog-to-digital converter 100 converts the respective analog RGB signals to digital data. A matrix converter 200 receives the respective digital RGB signals which have been converted from analog form to digital data, and output an intensity signal Y, and two chrominance signals (B-Y) and (R-Y), which are signals used in the NTSC system (or the PAL system) for television.
A vertical filter circuit 400 receives the horizontal scanning lines of the intensity signal Y of the NTSC system from the matrix converter 200, averages the intensity signal Y and outputs the averaged intensity signal Y for the NTSC system. A vertical filter circuit 400 further includes a horizontal filter circuit 420 and a selector 350. Delay memories 401 and 407 delay the respective averaged scanning lines of the intensity signal Y in the NTSC system by one clock cycle. An adder 402 adds three signals of the NTSC system, that is, the current intensity signal Y (=Y1), the intensity signal Y2 which has been delayed in the delay memory 401, and the intensity signal Y3 which has been delayed by two clock cycles by the delay memory 407. The adder averages the sum of the three signals.
The horizontal filter circuit 420 averages the chrominance signals (B-Y) and (R-Y) of the NTSC system in the horizontal direction. The horizontal filter circuit 420 averages a plurality of bits comprising the chrominance signals (B-Y) and (R-Y) of the NTSC system in the horizontal direction. The purpose of this averaging process is to suppress uneven color occurring in the horizontal direction. The selector 350 selects and outputs one of the averaged chrominance signals (B-Y) and (R-Y). A field memory 500 stores the respective intensity signal Y and chrominance signal C of the NTSC system, which have been output from the adder 402 and the selector 350, respectively, for one line. A write controlling circuit 550 controls writing of the intensity signal Y and the chrominance signal C of the NTSC system sent from the vertical filter 400 to the field memory 500. A read controlling circuit 560 controls reading of the intensity signal Y and the chrominance signal C of the NTSC system from the field memory 500. A conversion circuit 650 generates an intensity signal Y and the chrominance signals (B-Y) and (R-Y) from the intensity signal Y and the chrominance signal C output from the field memory 500. A digital-to-analog (D/A) conversion circuit 700 converts the digital intensity signal Y and the chrominance signals (B-Y) and (R-Y), received from the conversion circuit 650, to analog form. The results of this digital-to-analog conversion of the intensity signal Y, and the chrominance signals (B-Y) and (R-Y) of the NTSC system are provided to the television set. In this manner, the RGB signals in the VGA system are converted to the intensity signal Y and the chrominance signals (B-Y) and (R-Y) of the NTSC system, and the video data is displayed on the television set.
The operation of a conventional video system conversion circuit is explained below. In the video system conversion circuit constructed in the above-explained manner, the R signal (red signal), the G signal (green signal), and the B signal (blue signal) are input to the RGB input terminals 101, 102, and 103, respectively, of the analog-to-digital converter 100. The analog-to-digital converter 100 converts the respective analog signals into digital signals, and provides the digital signals to the matrix converter 200. The matrix converter 200 converts the respective RGB signals into the intensity signal Y and the two chrominance signals (B-Y) and (R-Y) of the NTSC system (or the PAL system), which is for the use in the television. The respective converted signals are output to the vertical filter circuit 400.
The intensity signal Y is input to the vertical filter circuit 400 from the matrix converter 200, and the intensity signal Y1 (=Y) is applied to the adder 402. The delay memory 401 delays the intensity signal Y1 by one clock cycle to generate the intensity signal Y2. The delay memory 407 delays the intensity signal Y2 by one clock cycle to generate the intensity signal Y3. The adder 402 adds these three intensity signals Y1, Y2, and Y3, and averages of the three intensity signals. The averaged intensity signal is delayed, and output as intensity signal Y. The chrominance signals (B-Y) and (R-Y) are input from the matrix converter 200 to the horizontal filter circuit 420. The horizontal filter circuit 420 averages of the respective input chrominance signals, and the results are output. The selector 350 selects one of the averaged chrominance signals (B-Y) and (R-Y) of the NTSC system which are input from the horizontal filter circuit 420, and the selected signal is output as a chrominance signal C.
The intensity signal Y and the chrominance signal C of the NTSC system are output from vertical filter circuit 400, and input to the field memory 500. The input signals are stored in the respective 1 H line memories. The writing of the intensity signal Y and the chrominance signal C of the NTSC system are performed under the control of the write controlling circuit 550. The written signals are read from the field memory 500 under the control of the read controlling circuit 560. The write controlling circuit 550 operates synchronously with a writing signal, and the read controlling circuit 560 operates by a read signal synchronously with the television signal.
The intensity signal Y and the chrominance signal C of the NTSC system are read out from the field memory 500, and input to the conversion circuit 650. The conversion circuit 650 converts the intensity signal Y and the chrominance signal C to the intensity signal Y and the chrominance signals (B-Y) and (R-Y), and the converted signals are output to the digital-to-analog conversion circuit 700. The digital-to-analog conversion circuit 700 converts the input digital signals into analog form. The converted intensity signal Y and the chrominance signals (B-Y) and (R-Y), which have been converted from the digital data to analog form are provided to the television set, and the television set displays these signals on the screen.
In the conventional video system conversion circuit as explained so far, since no thinning process has been performed on the intensity information Y and the chrominance signal C, or the intensity signal Y and the chrominance signals (B-Y) and (R-Y) in the horizontal direction, it is necessary to provide a field memory corresponding to one full line. Therefore, a large capacity field memory has been necessary so far.